As is well known, certain microwave oscillators and amplifiers commonly use a semiconductor diode, such as an IMPATT or a TRAPATT diode, as the negative resistance element of the circuit necessary for sustaining microwave or millimeter wave oscillations therein. Normally this diode is mounted at a selected location in the oscillator or amplifier waveguide structure and is provided with a suitable DC bias connection so that the diode will conduct current and sustain oscillations at millimeter wave or microwave frequencies when the diode impedance is properly matched to a load impedance. When using a two terminal negative resistance device such as an IMPATT diode, it is frequently desirable to provide two degrees of tuning for the circuit, which are commonly referred to as series and parallel tuning. This versatile tuning characteristic makes it possible to match the real and imaginary components of the diode impedance with those of the load impedance. The latter is desirable in order to achieve a maximum power transfer and a maximum DC to RF conversion efficiency for the oscillator or amplifier at a chosen frequency of operation.